Rolling is a metalworking process in which a material is passed through one or more pairs of working rollers in order to decrease the thickness of the slab and/or metal strip. The temperature of the material determines whether the rolling process is characterized as hot rolling versus cold rolling. Both hot rolling and cold rolling processes are commonly used.
Mill stands for rolling may be arranged in a number of configurations. For example, a four-high mill stand has four rollers, two smaller working rollers and two larger back up rollers. The material is pressingly engaged between the two working rollers as the two larger backup rollers engage the working rollers to stabilize the working rollers, which may facilitate greater accuracy of, for example, the gauge, width, or finish of the resulting rolled material.
It is imperative that the surfaces of the working rollers are free of mill marks because the surfaces of the working rollers are in direct contact with the material being rolled. Mill mark, as used herein, means an imperfection in the surface of the roller such as, for example, a dent, scratch, ding, or bruise skinning means creating a mill mark in the surface of the roller, and a skinned roller has a mill mark in the surface, as used herein.
A skinned working roller may imprint the mill mark into the resulting rolled material. Thus, when a working roller becomes skinned, the skinned working roller is removed from the mill stand and replaced with an unskinned working roller that is free of mill marks. The downtime required to replace a skinned working roller may range from about 15 minutes to about 20 minutes, which adversely affects productivity, efficiency, and profit margins. A grinding process that eliminates mill marks may be used in refurbishing the skinned working roller into an unskinned working roller. Refurbishing may cost from about US $7,000 to about US $10,000 per working roller.
An unskinned working roller may be skinned during replacement, which may lead to additional costs and down time. For example, in order to replace a set of working rollers in a four high mill stand, an overhead crane is engaged with the upper working roller. The crane lifts the upper working roller from the mill stand and onto a used roller rack. The crane then lifts the lower working roller from the mill stand and onto the used roller rack.
Once the skinned working rollers are removed from the mill stand, replacement working rollers free from mill marks may be mounted to the mill stand using the crane. For example, first an unskinned lower working roller is mounted to corresponding spindles of the mill stand. Then, an elongated rigid barrier, such as a wooden board, is placed atop the lower working roller to protect the lower working roller from being skinned. Finally, the upper working roller is mounted to a corresponding spindle of the mill stand.
The rigid barrier is supposed to prevent the working rollers from touching one another during mounting of the upper working roller onto the mill stand, as such touching may result in skinning of the working rollers. However, the working rollers may be greased with oil, thus making frictional engagement between the rigid barrier and the top of the lower working roller difficult during mounting of the upper working roller onto the mill stand. The rounded surface of the lower working roller may make the placement of the rigid barrier atop the lower working roller unstable. Vibrations caused by the crane during mounting of the upper working roller may dislodge the rigid barrier thereby rendering the rigid barrier non-functional. The rigid barrier may be dislodged during mounting of the upper working roller, for example, through contact with equipment or personnel or in other ways. When dislodged, the rigid barrier may no longer function, for example, to prevent touching between the working rollers that may result in the formation of mill marks.
It is also a present practice to put a second elongated rigid barrier atop the upper working roller following mounting of the upper working roller to prevent accidental touching between the upper working roller and the back up roller that may result in mill marks. Again, such use of the second elongated rigid barrier is unsatisfactory, suffering from deficiencies similar to the above described deficiencies of the rigid barrier in conjunction with the lower working roller. The rigid barrier(s) may suffer from deficiencies other than those described, as well.
Accordingly, there is a need for improved apparatus as well as related methods of use that prevents skinning of rollers during the mounting of rollers to a mill stand.